Add LICENSE, update .gitignore, add security note to README

- MIT LICENSE (2026 Thales Maciel)
- .gitignore: replace broad /build/ with explicit /build/bin/ and
  build/manual/ so large manual rootfs/kernel artifacts are clearly
  excluded; add *.pem, *.key, id_rsa
- README: add Security section documenting intentional
  PermitRootLogin yes / StrictModes no in guest sshd and the
  network boundary that makes it acceptable

README.md

@@ -280,6 +280,21 @@ Alpine support currently applies to the explicit register-and-run flow above.
 The generic `banger image build --from-image ...` path remains Debian/systemd-
 oriented and should not be treated as an Alpine image builder.
 
+## Security
+
+Guest VMs are single-user development sandboxes, not multi-tenant servers.
+Every provisioned image is configured with:
+
+```
+PermitRootLogin yes
+StrictModes no
+```
+
+This is intentional. The host SSH key is the only authentication mechanism,
+no password auth is enabled, and VMs are reachable only through the host
+bridge network (`172.16.0.0/24` by default). Do not expose the bridge
+interface or the VM guest IPs to an untrusted network.
+
 ## Notes
 
 - Firecracker is resolved from `PATH` by default.